1. Field of the Invention
The present invention relates to a digital programmable frequency generator.
2. Description of the Prior Art
Frequency generators that may be programmed are utilized in a wide variety of applications. Typical applications include radio and television receivers and transmitters, and computer devices that must operate at different clock rates, or be compatible with systems that operate at different clock rates. A typical prior-art technique for implementing a programmable frequency generator is the use of a phase-locked loop (PLL). The PLL includes a voltage-con,oiled oscillator (VCO), phase comparator, and programmable counter that are arranged in a well-known configuration. By changing the count of the counter, a desired frequency division ratio can be achieved that produces the desired output frequency from the VCO. However, a PLL has an inherent limitation as to the speed that a change in frequency can be made, which is dependent on the loop characteristics and the magnitude of the change. Furthermore, typical PLL implementations require significant power to operate, and their analog nature makes them susceptible to noise problems. In addition, many PLL designs are not readily adapted to low-voltage operation, as may occur, for example, in battery-operated potable systems where 3 volt (or less) operation is becoming common.
One technique for overcoming some of the PLL limitations is direct digital synthesis (DDS). In that technique, a digital representation of an analog waveform, for example a sinusold, is stored in read-only memory. By repetitively reading out the memory at a given ram, and applying the digital waveform to a digital-to-analog converter, an analog waveform at the desired frequency is obtained. The DDS technique typically allows for relatively rapid changes in frequency. However, DDS is overly complicated for many applications, as when only a binary clock waveform is needed, for example. It also may not lend itself to low-voltage, low-power applications. Another approach that is more oriented to digital technology involves the use of a ring oscillator with multiple inverter stages. The number of active stages is changed, in order to change the frequency of oscillation. However, that may require a large number of inverter stages to obtain a wide frequency range, especially if relatively small steps in frequency are required over the range.